While silver halide photographic materials are presently the most excellent means for recording optical information as image signals in terms of sensitivity and image quality, they are disadvantageous in that development processing is required and they cannot be repeatedly used for recording and erasure. Although an electrophotographic system using an electrostatic photoreceptor is capable of repeated image recording, the recording sensitivity is low, and the whole system is not suitable to be reduced in size as an image recording element. Photoelectric converting elements utilizing photoelectric charge separation of a semi-conductor, such as a CCD image sensor, are capable of detecting image information at a relatively high sensitivity, but the image quality is limited due to the large size of the picture element and, besides, the elements per se have no function of recording and maintaining an image. Further included in known optical recording systems is a system in which information is recorded by picture elements of molecular order size utilizing photochromism of organic dyestuff molecules. However, any of the so far developed photochromic recording systems has an extremely short time of information retention and poor contrast of developed image density and is, therefore, unsuitable for recording and display of a two-dimensional image.
On the other hand, a method for achieving high sensitivity recording in which image information is caught by rhodopsin, a photosensitive chromoprotein of visual pigments, as a photoreceptor on a molecular level and then displaced with a chemical change has been proposed as disclosed in U.S. Pat. Nos. 4,084,967 and 4,356,256 to D. F. O'Brien. However, a system of converting or amplifying optical information through a chemical reaction takes time for transmission of information and has low durability against repetition of recording and erasion of information.
While application of the technique of D. F. O'Brien is confined to formation of a monochromatic image, JP-A-1-116536 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a means for forming a color image by combining a photosensitive chromoprotein including rhodopsin with various pH-sensitive color forming reagents. According to this technique, since the color forming reaction is reversible with respect to a pH change, the chromoprotein must be fixed in an amphiphatic medium which is impermeable to hydrogen ion and also capable of retaining an appropriate water content for manifestation of the function of the protein, such as a lipid membrane, before a developed color image can be maintained at a stable pH condition. However, such a medium including a lipid membrane generally has a finite hydrogen ion osmotic coefficient and does not exhibit perfect impermeability. This fact is also revealed by a phenomenon that a light-induced pH change caused by rhodopsin fixed in a lipid vesicle returns to the original pH value in the dark in a short time (Reference can be made, e.g., in M. P. Heyn, FEBS Letters, Vol. 108, p. 359 (1979)). Accordingly, it has been demanded to establish a system reversibly responding to a pH change for maintenance of an image.
None of the above-described conventional techniques provides any suggestion for an optical recording system by which stable maintenance of image information, repetition of image recording and erasure, and high quality image recording using a photoreceptor of molecular size as a picture element can be simultaneously realized.
On the other hand, image sensors which are generally employed for detecting optical information as a two-dimensional image include two-dimensional area sensors using a solid-state junction element, such as a photo diode, an MOS transistor, and a CCD image sensor. Using a solid-state junction element as a picture element, these two-dimensional area sensors have a limit in size of picture elements (several microns or greater) from the technical consideration, and the density of optical information reception is accordingly limited. In order to detect optical information at a higher density, it has been demanded to reduce the size of picture elements to a molecular order.
As a system for recording optical information by picture elements of molecular size, an optical recording system of photon mode using an organic dyestuff compound as a recording medium has been under study in the field of laser photo discs, etc. For example, optical information recording systems utilizing photoisomerization and photochromism of dyestuff compounds are disclosed in JP-A-59-122577, JP-A-62-147453, JP-A- 61-117537, JP-A-64-9282, and Chemistry and Industry, Vol. 18, p. 193 (1978). Any of these systems provides a means for recording still information at a high density through one-dimensional scanning and is unsuitable for recording two-dimensional dynamic information as is feasible with a CCD image sensor.
A visual organ is the most advanced area sensor which can take up two-dimensional dynamic optical information via a picture element of molecular size level. In the visual organ, rhodopsin is a photosensitive chromoprotein which functions to detect optical information. Various studies for reproducing this function of rhodopsin in vitro for constructing a photo sensor have been reported. For example, O'Brien suggests converting photo behavior of the visual pigment, rhodopsin, to changes in chemical amount to thereby achieve recording with high sensitivity as disclosed in U.S. Pat. Nos. 4,084,967 and 4,356,256. Further, JP-A-1-116536 discloses a technique for displaying a color image information input by using a combination of a proton-transporting photosensitive chromoprotein, such as bacteriorhodopsin, with various pH-sensitive color forming reagents.
In any of these photo sensors, since optical information is converted to a chemical reaction amount, the reaction product is accumulated with time of light irradiation. Therefore, it is difficult to display the output and obtain reversible on-off response in accordance with changes in intensity of light.
Various photoelectric conversion systems using an oriented film of rhodopsin have been proposed as a means capable of rapidly responding to dynamic information as a photo sensor. A photovoltaic sandwich cell using a bacteriorhodopsin film is a typical example of such photoelectric conversion systems. Examples using various oriented films are shown, e.g., in K. Nagy, Biochem. Biophys. Res. Commun., Vol. 85, pp. 383-390 (1978), G. Varo, Acta Biol. Acad. Sci. Hung., Vol. 32, pp. 301-310 (1981), and T-Furuno, et al., Thin Solid Films, Vol. 160, pp. 145-151 (1988). Further, a photo sensor by which proton transportation of a bacteriorhodopsin film in water is detected by using a pH-sensitive ISFET as an electrode is disclosed in JP-A-59-197849 and JP-A-61-11158.
In the above-described conventional photo sensors, if a solid-state junction element, such as a CCD image sensor, is employed, size reduction of picture elements is difficult. Optical recording systems utilizing photochromism of organic molecules, on the other hand, have a reduced picture element size but are unsuitable for monitoring dynamic information due to low optical response speed, particularly in dark reaction for return. Further, photoelectric conversion type photo sensors using photosensitive chromoproteins such as bacteriorhodopsin are capable of rapidly detecting dynamic optical information such as a change in electric quantity corresponding to on-off switching, but are hardly capable of achieving conversion of optical information on a molecular size level due to limitation of the picture element to the size of the electrode just like a CCD image sensor.